It is known practice to use a radar function for detecting non-cooperative airborne objects for aircraft, in particular to equip drones. This function is essential to allow for the insertion of auto-piloted aircraft in the non-segregated air space. It participates in the obstacle avoidance function, known by the name “Sense and Avoid” (S&A).
Such a radar needs to have a very wide field of observation (typically ±110° in azimuth and ±15° in elevation) and must be capable of scanning the space in a very short time, of the order of a second, given the time it takes to undertake an avoidance manoeuvre in the presence of collision risks. These characteristics correspond approximately to the environment observation capability of a “human” pilot (“See and Avoid”). For such an application, it is advantageous to use one or more antennas with wide transmission field, a plurality of antennas with little directivity in reception mode and to form, in reception mode, multiple beams simultaneously in the illuminated space: this is the technique known to those skilled in the art by the name “Beam Formation by Calculation” or “BFC”. This solution is conventionally implemented using planar arrays of antennas for which the patterns formed need to have a directivity that is sufficient to locate the targets with good accuracy.
Some of the above points can be resolved by the solutions described in the patent applications FR09 04395, FR09 04224, FR09 04394 and FR09 04880.
In the Patent Application FR09 04395, the general problem of the radar system is resolved by introducing, in its preferred solution, a double waveform managed adaptively:                the fast targets are detected at a great distance by a waveform without Doppler ambiguity where these fast targets can be detected on thermal noise after Doppler filtering;        the near or slow targets in clutter are detected by a waveform adapted to the visibility in ground clutter:                    it is not ambiguous, either in distance or in speed (the targets concerned are near and slow).            It is preferably a waveform with “high” distance resolution (distance-separating power).                        
In the Patent Application FR09 04224, the problems linked to the antenna system are resolved by using, in its preferred solution, a “separate antennas in cross” configuration associated with a principle of “coloration” of the space in transmission and with the use of the Beam Formation by Calculation (BFC) in reception. The coloration of the space involves transmitting N orthogonal signals on N transmitting antennas. These signals are then separated by filtering on reception given their orthogonality properties. It is, for example, possible, with two contiguous antennas in transmission associated with two or more orthogonal codes to produce, in transmission, the functional equivalent of a “single-pulse” angular measurement system.
In the case of the preferred solution described in the Patent Application FR09 04224, the two waveform generators (WFG) are successively connected to three arrays, one array consists of a pair of antennas oriented towards three different bearings to cover the space in bearing mode. This patent application proposes an alternative to the use of three switched arrays by introducing, notably in the “transmission” part, a discrete electronic scanning in three directions by virtue of switches acting at low level (before the power stage). This switching may employ, for example, delay lines short-circuited by PIN diodes or MEMS devices.
The Patent Application FR09 04394 notably describes waveforms and the signal processing principles to best exploit the antenna configuration described in Patent Application FR09 04224 and by taking into account the adaptive nature of the choice of the waveforms described in the Patent Application FR09 04395.
The Patent Application FR09 04880 improves on the angular measurement accuracy of the system described in Patent Application FR09 04224. This solution notably consists in simultaneously associating:                an interferometer that is accurate but ambiguous in reception;        a space coloration principle in transmission.        
It is the combination of the angular measurements obtained from the interferometer, accurate but ambiguous, and of the space coloration system which ultimately makes it possible to obtain accurate and unambiguous angular measurements by virtue of the so-called “single pulse in transmission” system, with the coloration in transmission, which is used to measure at least the ambiguity rank of the angular measurement supplied by the interferometer.
An exemplary embodiment according to the abovementioned patent applications was presented at the “IRS 2010” conference on 17 Jun. 2010 in “A MIMO Radar for Sense and Avoid Function—A fully static solution for UAV” by Stéphane KEMKEMIAN et al.
If the receiving antennas are linear arrays, their coverage in azimuth is generally limited to ±60° relative to the normal to the array, or even more, if a performance degradation is accepted. This coverage is insufficient in light of the need expressed previously (coverage of ±110° in azimuth). In these conditions, two preceding devices are needed, each covering at most only approximately ±60° and each being oriented towards a particular direction.
These two devices can work alternately or else simultaneously:                In the first case, each system works only 50% of the time. All other things being equal, a loss of 3 dB ensues due to the reduction of the signal integration time;        In the second case, assuming that the conditions necessary for the simultaneous operation are satisfied, there is no loss on each of the two devices, but the transmitted power is doubled. If a constant total transmitted power (therefore constant consumption on the onboard electrical network is assumed), there will also be a loss of 3 dB.        
Furthermore, in both cases, the bulk on board of the drone is doubled.
One aim of the invention is to retain the same energy efficiency:                by accepting a size increase only for the receiving antennas;        by retaining a single transmitting antenna;        by increasing the angular coverage of a single system up to ±110° in azimuth.        
The result is a single radar and not a doubled system as in the preceding case where the receiving antennas are perfectly linear.
The vertical locating principle described notably in the Patent Application FR09 04880 is retained. The same applies regarding the use of the Beam Formation by Calculation in the horizontal plane described notably in the Patent Applications FR09 04224, FR09 04394 and FR09 04880.